Larger Tires 'on 99 3326

The wheel wells on my 1999 3326K look large enough to put 14" tires on the trailor. It now has 13". The 14" wheels only cost about $25-35 apiece. I often tow at high speed for long distances (75 MPH) and my 13" Goodyear tires are cupping. I bought used so it may also be caused by low presure at some earlier time. What size 14" tires does the factory use on some models? Is this a bad idea. The extra height won't hurt as the trailer is a little low for my truck.